Klass.inherited(sub_klass) and Klass.clone() - to ensure that the
Singleton pattern is properly inherited and cloned.

Klass.instance() - returning “the instance''. After a successful
self modifying (normally the first) call the method body is a simple:

defKlass.instance()return@__instance__end

Klass._load(str) - calling Klass.instance()

Klass._instantiate?() - returning “the instance'' or nil. This
hook method puts a second (or nth) thread calling Klass.instance() on a
waiting loop. The return value signifies the successful completion or
premature termination of the first, or more generally, current
“instantiation thread”.

The instance method of Singleton are

clone and dup - raising TypeErrors to prevent cloning or duping

_dump(depth) - returning the empty string. Marshalling strips by default
all state information, e.g. instance variables and taint state, from “the
instance''. Providing custom _load(str) and _dump(depth) hooks
allows the (partially) resurrections of a previous state of “the
instance''.